1. Field of the Invention
This invention relates to a cold-setting starch adhesive which is suitable for use in the manufacture of corrugated fibreboard.
2. Description of the Prior Art
Starch adhesive has been commonly used in the manufacture of corrugated fibreboard. Known starch adhesives generally consist mainly of ungelatinized starch paste. Thus, they are used by Stein-Hall process. That is, after the starch paste is firstly gelatinized to sticky paste by steam heating and treatment with caustic soda solution, it is successively applied to the tips of corrugated medium to be bonded to the liner to form corrugated fibreboard, and subsequent heating of the corrugated fibreboard vaporize the water from the paste to dryness to complete the setting.
The heat energy consumed by the corrugating operation of Stein-Hall process occupies the major part of the total energy consumption in corrugated fibreboard manufacturing plants. If the heating procedure can be eliminated from the corrugating operation, it will contribute to energy saving in corrugated fibreboard manufacturing plants. It has, therefore, been of great interest to the corrugated fibreboard industry to develop a cold-corrugating process which does not require any heating procedure at the corrugating step of corrugated fibreboard.
Some hot-melt adhesives obtained as petrochemical product is known as being cold-setting. They, however, have the following disadvantages. The raw materials of the hot-melt adhesives are becoming less available because of the high price of petroleum. Used corrugated fibreboard made by employing a hot-melt adhesive is difficult to recover and reuse. Accordingly, it has been strongly desired to develop a cold-setting corn starch adhesive for use in the manufacture of corrugated fibreboard.
Certain properties are required for a cold-setting corn starch adhesive which is to be used for bonding in the manufacture of corrugated fibreboard without heating procedure. That is, the paste solution must have high solid content, so that it may contain only a small quantity of water to be vaporized. It must be able to gel rapidly, as it is required to set immediately after the application to the fibreboard surfaces to be bonded together.
Other properties are also required for an adhesive for use in the manufacture of corrugated fibreboard, namely, during the manufacture of corrugated fibreboard, production speed of as high as possible is desired on a corrugator in normal operation, while it must be slowed down when a small lot of products is manufactured, or at the beginning or the end of the operation. It is, therefore, necessary to employ an adhesive having a constant gluing ability irrespective of the production speed, whether it may be high or low. This requirement is nowadays of increasing importance, since the operation speed on a corrugator is as high as 250 meters per minute.
However, ordinary corn starch adhesives, when they are used as cold-corrugating adhesives, tend to gel so slowly that unsatisfactory setting appears if the operating speed of the corrugator is increased, and even a slight external force can cause the separation of the glued surfaces when, for example, the corrugated fibreboard is cut to a prescribed size. Therefore, when they are used as cold-corrugating adhesives in the process of manufacturing corrugated fibreboard, the efficiency of the process is far inferior as compared when they are used in Stein-Hall process.
Japanese Laid-Open Patent Specification No. 32570/1981 discloses a cold-corrugating adhesive for corrugated fibreboard having improved glueability at high production speed. This adhesive consists mainly of high amylose corn starch instead of ordinary corn starch which is the main component of all the known starch adhesives. High amylose corn starch is a starch prepared from specialty corn created by selective breeding. It contains at least 50% of amylose, while ordinary corn starch contains only about 25% of amylose.
Because of its high amylose content, high amylose corn starch has a variety of outstanding properties, for example:
(a) It is hard to be gelatinized under normal conditions;
(b) it can form high solid paste;
(c) it is highly susceptible to retrogradation;
(d) it forms strong film; and
(e) it has high glueability.
Some of these properties of high amylose corn starch make it a very suitable substance which can impart rapid glueability to cold-corrugating adhesive for use in the manufacture of corrugated fibreboard. As already pointed out, paste prepared from cold-corrugating starch adhesive must have high solid concentration when it is used at higher operating speed so that it may contain only a small amount of water to be vaporized, and must be capable of gelling rapidly after the application to the fibreboard surfaces to be bonded together. These requirements are fully satisfied by the properties (b) to (e) of high amylose corn starch.
The high amylose corn starch adhesive disclosed in Japanese Laid-Open Patent Specification No. 32570/1981 is, thus, partly satisfactory for application at higher operating speed. It, however, retrogrades and dries too rapdily, and can retain only a small amount of water. Consequently, if the operating speed is low, it is likely to solidify on the applicator roll, or on the fluit tip of the corrugating medium before bond it with the liner. Thus the adhesive from high amylose corn starch cannot show satisfactory glueability outside of only a narrow range of operating speeds. The use thereof for application at lower operating speed is likely to result in an increased percentage of defective bond.
Another problem resides in the quality of the material which is available for making corrugated fibreboard. Recent shortage of pulp resources makes it difficult to obtain good pulp, and necessitates the use of fibreboard material of lower strength for making corrugated fibreboard. Synthetic resins, or other reinforcing agents are often added to the material in order to compensate the deficient strength. Synthetic resins are sometimes added purely for the purpose of producing reinforced liner or medium having improved strength irrespective of the quality of the pulp. The adhesive prepared from high amylose or ordinary corn starch is, however, very unsatisfactory for gluing any such resin-reinforced liner or medium.
Under these circumstances, it has hitherto been considered impossible to produce from starch a cold-corrugating adhesvie for corrugated fiberboard which shows high glueability both at higher and lower operating speeds.